Investments (Unrealized Losses) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Investment securities available for sale:
Less than 12 months, fair value	$ 5,340,271	$ 1,173,274
Available for Sale Securities Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses Accumulated In Investments	24,777	2,203
Aggregate fair value, continuous unrealized loss position, 12 months or more	280,126	832,420
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	6,094	10,043
Total fair value	5,620,397	2,005,694
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	30,871	12,246
U. S. Treasury
Investment securities available for sale:
Less than 12 months, fair value	1,539,637	338,612
Available for Sale Securities Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses Accumulated In Investments	1,118	151
Aggregate fair value, continuous unrealized loss position, 12 months or more	0	1,015
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	1
Total fair value	1,539,637	339,627
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	1,118	152
Government Agency
Investment securities available for sale:
Less than 12 months, fair value	229,436	261,288
Available for Sale Securities Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses Accumulated In Investments	374	247
Aggregate fair value, continuous unrealized loss position, 12 months or more	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Total fair value	229,436	261,288
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	374	247
Mortgage Backed Securities, Other
Investment securities available for sale:
Less than 12 months, fair value	3,570,470	573,374
Available for Sale Securities Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses Accumulated In Investments	23,275	1,805
Aggregate fair value, continuous unrealized loss position, 12 months or more	280,126	831,405
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	6,094	10,042
Total fair value	3,850,596	1,404,779
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	29,369	11,847
Equity Securities
Investment securities available for sale:
Less than 12 months, fair value	728
Available for Sale Securities Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses Accumulated In Investments	10
Aggregate fair value, continuous unrealized loss position, 12 months or more	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Total fair value	728
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	10
Other Debt Obligations
Investment securities available for sale:
Less than 12 months, fair value	0
Available for Sale Securities Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses Accumulated In Investments	0
Aggregate fair value, continuous unrealized loss position, 12 months or more	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Total fair value	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	$ 0
State, County and Municipal
Investment securities available for sale:
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments		$ 0